I just received a Gillette Bostonian that has a 1/2 inch crack on the barrel up by the head. Is it safe to shave with this razor or will I ruin the razor?
You should be able to shave with it. However, many people have used either an epoxy or solder to repair it. I can't say what type of solder since I haven't done it myself. Sent via mobile - Chris
Welcome to The Shave Den! I'm glad you're here. I had a Gillette New Long Comb with a similar crack on the handle. I shaved with it for awhile, but the head would occasionally move. I eventually sent the razor off to have the crack soldered and the entire razor recoated. No problems at all with it now.
It is safe to use it. When buying a vintage razor it's quite common for it to have a crack in the barrel. Like mentioned above you can solder or use an epoxy. If it's a 3 piece you can buy a different handle for it. Good find. They're not super rare but also not real common.
I have found the New Improved, and the Single Ring style Gillette razors stay together just fine with small cracks in the outer barrel. However, the Ball End type handles with cracks can draw blood. Just a few weeks ago one of my prized US Army razors with a crack in the upper part of the barrel came apart in mid shave. It was due to the crack getting wider, causing the threaded tip to come out, while I was shaving. It drew blood. It's the Ball End handles you have to worry about mostly.
...And if you see one on eBay with a suspiciously nice uncracked handle, chances are the cracked handle was replaced by a later ball end tech handle. It arguably makes the razor shave better, but ruins the value of the razor.